I have a 98 yz400. Runs great ecxept for the off idle bog. Its not bad but it does bother me. I would love to get this fixed if possible. I live in TN and ride from o to 6000ft. I currently have a 170 main jet a 48 pj I have done the taffy mod and played with the fuel screw to many times to count. Everything else in stock. I have tried a 175 and 180 mj. I have also tried a 45 pj. I also have a wirtz acc pump cover. Anyone ever get this worked out or is it not possible? Anyone have any jetting suggestions? Thanks guys

I had it too after a winter carb cleaning and adding a WB E series - I got the jetting all dialed in but the bog was continuous - I turned the AP timing screw in just a hair (maybe 1/8th of a turn and it completely eliminated it - likely advanced my AP squirt just a split second - it was enough to make a huge difference.

I'm wondering if there wasn't possibly just enough grime buildup on the screw end, the AP push rod ends and the AP diaphram (where the pushrod touches) that once cleaned off changed the AP timing just that tiny bit that I had to adjust the screw........seemed to make sense to me at teh time since the whole AP control circuit was pretty filthy.

What jetting are you guys running on the yz400

Mine's a WR so Jetting will be different.

i have a 180 MJ and the stock PJ. i think the problem is the AP. either it needs to be replaced, or needs to squirt a tiny bit earlier like matt4x4 did.

what is the ap screw

Ok I know what it is now but I dont think the 98400 has it. If it does can someone show me a picture.


I also have a 98 yz400f. I had the same problem. I tried everything,jetting,fuelscrew,but in the end I replaced my ap diaphram. and it took care of all my bog.

did you get the stock one or a different size one

I have a 2001 YZ426 and had the same problem specially when climing a hill plus it would take forever to start. So I sent my carb to Zip-ty racing and they did their mods for 120 bucks and bamm! My bike starts on the first kick with no hesitation what so ever. This is by far the best mod I have ever done for my bike and I recommend this to anyone with an older bike. Just my two cents, hope it helps.

I replaced with the stock size diaphram,I think it's around $20

I will try that first. The diaphram looked like it needed to be replaced anyway.

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